Refrigeration systems with forced ventilation, which are usually applied in refrigerators and freezers, generally use an evaporator of the compact tube-fin type, comprising a plurality of fins that are incorporated and trespassed by a bundle of tubes arranged in series in the form of a coil, inside which flows a refrigerating fluid. Externally to the tubes and to the fins of the evaporator passes a forced airflow, which is removed from the inside of an environment to be refrigerated so as to be refrigerated by the evaporator and discharged back to the inside of said environment, as it occurs in the refrigerating or freezing compartments of a refrigeration appliance, for example.
These evaporators are constructed so as to assure a determined heat exchange rate between the forced airflow passing through the tubes of the evaporator and through the fins externally affixed to the tubes.
The evaporators of the tube-fin type with forced ventilation are normally used in household refrigerators and freezers of the no-frost or frost-free types with automatic defrost, usually comprising two vertical and parallel rows of horizontal tubes 20, which are incorporated to fins 30 and connected in series in each row, defining two mutually parallel vertical coils connected in series to each other, as shown in FIGS. 1, 2, and 3 of the enclosed drawings. In these evaporators, the refrigerating fluid, which is refrigerated in the condenser of the refrigeration system and expanded through an expanding device, is supplied to a tube 20 of a first vertical coil S1 located in the region 12 for the outlet of the forced airflow F that passes through the evaporator 10. The refrigerating fluid flows through the first coil S1, generally from top to bottom and in an opposite direction or in counterflow in relation to the forced airflow F.
The refrigerating fluid is conducted to a second coil S2, flowing through the latter in an opposite direction to the flow in the first coil S1, that is, in the same direction of the forced airflow, defining a parallel or concurrent flow heat exchanger.
The prior art constructive arrangement utilizes two parallel coils (S1, S2) connected in series and conducting the refrigerating fluid in opposite directions, that is, defining, respectively, a counterflow heat exchanger followed by a heat exchanger with a flow that is parallel to the forced airflow passing through the evaporator.
In the constructions of this type, in which it is possible to occur a significant overheating of the refrigerating fluid in the region close to the outlet of the evaporator, which situation generally occurs during the transient working period, the first coil S1, which operates as a counterflow heat exchanger, presents a higher efficiency than the second coil S2 operating as a parallel flow heat exchanger.
Considering that the operation of the household refrigerators and freezers is controlled by a thermostat, the behavior thereof is typically transient, making the tubes 20 of the evaporator 10 contain overheated vapor during most of its working period.
If there were no overheating of the vaporized refrigerant at the outlet of the evaporator, the second coil S2 in concurrent flow in relation to the forced airflow F would present a performance equivalent to the first coil S1 operating in counterflow.
However, the condition without overheating is observed only when the refrigeration system is working in a permanent regimen, which does not occur in practice.
The fact of existing a region with overheated vapor in the evaporator 10 of a refrigerator or a freezer with forced ventilation during most of the working time of these household appliances indicates that the performance of the first coil S1 in counterflow is better than that of the second coil S2, in which the flow of the refrigerating fluid is parallel to the forced airflow and in which the overheating region generally appears (outlet region).